In general, a liquid crystal display device comprises an array substrate including a switching element and a pixel electrode, a counter-substrate opposed to the array substrate, and a liquid crystal layer interposed between the array substrate and the counter-substrate.
In the liquid crystal layer of the liquid crystal display device, there are ions induced by elements provided in the array substrate and the counter-substrate or by impurities entering from portions attached to the array substrate and the counter-substrate. These ions move through the liquid crystal layer, for example, as the liquid crystal display device is driven.
If the ions are concentrated into a certain portion in the liquid crystal layer, the effective voltage applied to the liquid crystal layer will decrease in this portion. As the effect voltage decreases in this portion, the luminance of the portion decreases accordingly, and this will have an effect such as a black spot on a display image.